Q3(a). Two transformations: indole and 1-vinylcyclohexene

Reading of the question: the Hindi text asks for the mechanism, the English for the transformation; the reagents of each step are given with the key mechanism.

Introduction. (i) Electrophiles attack indole at C-3, so a C-2 substituent needs a different approach: protect N, lithiate C-2 (directed metalation), and use ethylene oxide as a two-carbon electrophile. (ii) The product has one more carbon than the diene (C₈H₁₂ → C₉H₁₄O); a cyclic hydroboration of both C=C bonds followed by carbonylation (Brown) stitches in the C=O as the ring carbon.

(i) Indole → 2-(1H-indol-2-yl)ethan-1-ol

1. NaH, then PhSO2Cl (THF): N-phenylsulfonylindole. The N–H is removed so that the base does not simply deprotonate N, and the sulfonyl group directs lithiation to C-2 by coordination and induction.
2. LDA (or n-BuLi), THF, -78°C: 2-lithio-1-(phenylsulfonyl)indole (C-2 H is the most acidic ring C–H).
3. Ethylene oxide (with BF₃.OEt₂), then H₃O⁺: SN2 ring opening at the epoxide CH₂ gives the 2-(2-hydroxyethyl) side chain.
4. NaOH or KOH in methanol, reflux: removes the sulfonyl group (N–S cleavage), giving the product.

Scheme: Directed C-2 lithiation, epoxide opening, N-deprotection.

(ii) 1-Vinylcyclohexene → octahydro-1H-inden-1-one (hydrindan-1-one)

1. Thexylborane (ThxBH₂, from BH₃ and 2,3-dimethylbut-2-ene), THF, 0°C. The first B–H adds to the terminal vinyl CH₂; the second adds intramolecularly to the ring C=C, B to C-2 (anti-Markovnikov), syn. A five-membered boracycle (borolane) fused to the cyclohexane is formed.
2. Carbon monoxide (about 70 atm, 50°C, in the presence of water): CO adds to boron, and both ring carbons migrate in turn from B to the CO carbon (the thexyl group migrates last and is kept back).
3. Alkaline H₂O₂: oxidises the C–B bonds, releasing the ketone. The CO carbon is now the C=O between the old vinyl CH₂ and ring C-2: a cyclopentanone fused to the cyclohexane.

Scheme: Each B–C bond migrates with retention, so the trans ring fusion of the borane is kept.

Stereochemistry: hydroboration places H (on C-3a) and B (on C-7a) cis; the two ring-fusion H atoms are therefore trans, and migration with retention gives trans-hydrindan-1-one.

Conclusion. (i) N-sulfonylation, C-2 lithiation, ethylene oxide, then N-deprotection. (ii) Cyclic hydroboration with thexylborane, carbonylation with CO and oxidation with H₂O₂/OH⁻: trans-octahydro-1H-inden-1-one.

Application. Directed lithiation is how C-2 substituted indoles (tryptamine and indole alkaloid analogues) are made; Brown's diene-to-cyclic-ketone route builds the hydrindanone (C/D ring) unit of steroids.